The present invention relates to heat exchangers and pertains particularly to heat exchangers for gas turbine engines and the like.
In the last thirty years, numerous efforts have been made to resolve the problems of heat exchangers for vehicular gas turbines. About forty years ago, Rover Company in England used gas turbines to propel passenger automobiles. During that period, aviation gas turbines, from jet engines to turbo fans, achieved unprecedented reliability and performance records and established the "Jet Era", which is still unmatched by any other type of transportation.
Gas turbine research programs and several experimental engines proved that for many industrial, vehicular, marine and off-highway applications that gas turbine is a more attractive prime mover than the Diesel engine. Many experimental applications showed that the recuperative gas turbine is the most fuel effective power plant available, and its development is important because of our growing energy requirements.
There are strong reasons why vehicular gas turbines, which worked with reasonable reliability and durability, were built by aircraft engine manufacturers or aerospace firms. However, the cost of manufacturing was so high that, due to competition with the Diesel and spark ignition engines, they could not be marketed.
Automotive or truck engine manufacturers also tackled the problem. However, their low cost technology was more suited to the mass production of Diesel and spark ignition engines, than to the more sophisticated manufacturing requirements of the gas turbine. In addition, their lack of expertise in gas turbine engineering was also an important factor in failure to produce a reliable engine.
In all cases, the most significant problem area seemed to be the heat exchanger. All of them were designed with very small hydraulic diameters, and fluids that passed through the heat exchanger had to be very large to make up for the "adequate" effectiveness.
The recuperative heat exchangers, if they are built with the current "state of art" using plate-fin or fixed boundary metallic recuperators, are too large and do not permit vehicular installation within the available engine envelope. They are all built with small hydraulic diameters (i.e. cross sectional flow areas), and have the tendency to cause high pressure drop and plug up with exhaust emission materials. This static recuperative heat exchanger is a very primitive structure and is not capable of handling high velocity fluids, the high pressure compressor discharge air, and the high temperature exhaust gas carrying combustion by-products.
Periodical (rotary) heat exchangers or regenerators show high thermal efficiency at small size, but present serious mechanical problems at the seals. Either the leakage rate is too high or the rubbing seals do not last long enough. Lubrication is impractical because of high temperature (1400 to 1800 degrees F.), and the cyclic radial and axial deflections of the rotating discs present unresolvable engineering problems. The periodical heat exchanger is really a heat storing device invented fifty years ago for air preheating of steam engine plants. Its inherent design characteristics cannot handle high pressure and high temperature fluids.
Because of these factors, virtually all vehicular gas turbine manufacturing efforts have comprised of endless development programs, which to date private industry cannot afford to support and finance.
It is therefore desirable that improved heat exchangers for high velocity gases be available.